This invention relates to an electroless immersion process for plating a tin bismuth solder alloy, and, more particularly, to such electroless plating process for plating a tin bismuth alloy comprising greater than 50 weight percent tin and preferably greater than 70 weight percent tin.
In the manufacture of microelectronic packages, it is known to plate a solder alloy onto a metallic surface of a printed circuit board or similar substrate. Solder plate is applied, for example, to a bond pad of a copper circuit trace in preparation to attaching a component by a surface mounting operation. A typical solder for this purpose is formed of a low-melting, near-eutectic tin-lead alloy. One process for forming the solder plate involves electrodeposition. The substrate is immersed in an aqueous solution containing tin ions and lead ions and is cathodically biased to reduce the ions to metal and co-deposit tin and lead metals. This technique requires electrical connections to the substrate to distribute the plating current. Alternately, the solder may be deposited electrolessly by immersing the substrate in a solution containing tin and lead compounds and also containing an agent effective to reduce the tin and lead at the metal surface. A preferred process utilizes an acidic solution comprising tin methane sulfonate, lead methane sulfonate, and thiourea. Electroless deposition has advantages by avoiding the need for electrical connections and in producing a more uniform plate thickness regardless of the configuration of the underlying metal surface.
For some applications, it is desired to employ a lead-free solder formed of an alloy of tin and bismuth, in contrast to the tin-lead solder alloy. Alloy containing a relatively high tin content, preferably greater than 70 weight percent, is preferred to provide a high ductility to minimize crack formation, while also comprising sufficient bismuth to reduce the melting point to facilitate subsequent soldering operations. Heretofore, an electroless deposition process has not been available for plating such tin bismuth alloy. This is attributed to the fact that, unlike tin and lead which require the addition of an agent such as thiourea to deposit the metal, bismuth deposits spontaneously onto copper and forms a deposit that is porous and has poor adhesion to the underlying copper, as evidenced by spalling. Thus, there is a need for an electroless immersion process for plating tin bismuth alloy onto a copper surface that produces a dense, adherent plate containing tin and bismuth in proportions suitable for soldering operations.